Various types of batteries are known. Among them, nonaqueous electrolyte batteries such as lithium ion batteries as well as nickel cadmium batteries, nickel hydrogen batteries, and lead battery have been increasingly used as batteries that have large energy densities.
Regarding the different structures of batteries, lamination type batteries as well as cylindrical batteries and square batteries are known. A cylindrical battery has a structure in which a cylindrically-shaped battery element that is composed of belt-shaped positive and negative electrodes and that is wound through a separator is contained in a battery case. A square battery has a structure in which such a wound battery element is flattened and contained in a square battery case. A lamination type battery has a structure in which a plurality of planar positive and negative electrodes are laminated through respective separators and current collection tabs connected to respective electrodes are connected in parallel.
In a battery having a winding type battery element using belt-shaped positive and negative electrodes, the battery element can be easily produced in such a manner that current collection tabs are connected to a negative electrode and a positive electrode and then the separator, negative electrode, separator, and then positive electrode are successively laminated and wound. On the other hand, such batteries have a problem in which, when the separators, negative electrode, and positive electrode are wound, they may be bent, active mass layers of the positive electrode and negative electrode become thick at sections where the radius of curvature becomes small and thereby current concentrates in these areas. In addition, since force that acts in the reverse direction of the winding direction occurs, it is necessary to prevent the winding type battery element from returning to their original positions. In addition, when a winding type battery element needs to be charged or discharged with a large amount of current, a plurality of current collection tabs are connected to the electrodes and wound. In this case, since the shape of the wound battery element becomes distorted, a problem sometimes occurs in the connection between the current collection tabs and the external connection terminals.
On the other hand, in a lamination type battery in which many planar electrodes are laminated, when the battery is charged or discharged, the volume of active mass varies only in the lamination direction. Such a volume change does not largely affect the battery. In addition, since the current collection tabs formed in the individual electrodes of the battery element can be accurately and easily connected, a small battery having a low current capacity can be easily produced. Moreover, since current collection tabs that have a large current conduction area can be used, a large battery that is charged and discharged with a large amount of current can be easily produced. As a result, a lamination type battery can be widely applied to batteries of various sizes.
A bag-shaped separator that coats a planar electrode has been used as a separator that separates adjacent planar negative electrode and positive electrode. When such a bag-shaped separator is produced, a microporous film made of a synthesize resin such as polyethylene or polypropylene is formed in a bag shape, a positive electrode or negative electrode to which a current conduction and connection tab is connected is contained in the bag shaped film, and then the opening section of the bag is closed by heat fusing. Although the bag-shaped separator becomes an effective means for preventing active material from peeling off from a planar electrode contained in the bag separator and dispersing, when the opening section is closed by heat fusing, the bag-shaped separator sometimes becomes wrinkled.
Thus, when a lamination type battery in which many bag-shaped separators are laminated is produced, it is likely that, since the separators become wrinkled and thereby the distance between laminated electrodes partly increases, various types of characteristics such as heavy load characteristic, cycle characteristic, and low temperature characteristic of the battery deteriorate.
If a heat fusing and joining section are equally and entirely formed on the periphery of the bag-shaped separator, it is likely that the strength of the heat fusing and joining section or its neighboring sections will weaken and thus the inner electrode will cause the bag to break.
To solve such a problem caused by the bag-shaped separator, a technique for forming the heat fusing and joining section on a part of the separator, but not on the entire area of the separator, has been proposed in a related art reference. Another technique that fuses and joins a separator at positions symmetrical with respect to the center line of an electrode has been proposed in another related art reference.
For example, Patent Literature 1 (JP7-272761A publication) describes that two separators sandwich one or more electrode plates from among a positive electrode plate and a negative electrode plate and the two separators are fused and joined together at predetermined intervals on the periphery of the positive electrode plate or negative electrode plate.
Patent Literature 2 (JP10-188938A, Publication) describes that in a secondary battery in which positive electrode plates and negative electrode plates are alternatively laminated through respective separators, two separators sandwich a positive electrode plate or a negative electrode plate and the two separators are fused and joined together at positions on the periphery with respect to the center line of the positive electrode plate or negative electrode plate including four corners of the positive electrode plate or negative electrode plate.
Next, with reference to the accompanying drawings, an ordinary lamination type secondary battery will be described more specifically. FIG. 1 is a plan view showing an ordinary lamination type secondary battery. Positive electrode 2 or negative electrode (not shown) that composes a battery element of the lamination type battery is coated with bag-shaped separator 4. Bag-shaped separator 4 has a construction in which the electrode is sandwiched by two separators made of a microporous film such as polyethylene or polypropylene and fusing and joining section 6 that is intermittently formed along the outer periphery of the electrode joins the two separators so as to form them into a bag shape.
However, in the related art reference shown in FIG. 1, a section that has the strength for a baseline based on which bag-shaped separators are laminated is not present on the outer periphery of each bag-shaped separator. Thus, although the alignment accuracy of an electrode in one bag-shaped separator can be adjusted, when positive electrodes, separators, and negative electrodes are laminated for a plurality of pairs, there is no improvement in the accuracy for laminating the separators, and thereby it is likely laminating dislocation will occurs in separators.
When a battery case is used to contain a battery element, if laminating dislocation occurs in separators, the battery element becomes thick and the battery element may not fit the battery case. This problem may occur in the case that a laminate film is used as a member that covers a battery element. In this case, it is possible to contain the battery element in such a manner that the section of the laminate film that contains the battery element has a tolerance for laminating dislocation. However, in this case, a problem in which energy density per unit volume decreases will occur.